Double fed induction generators (DFIGs) are commonly used in wind energy conversion systems (WECs) for interfacing a wind-driven turbine with an electrical power grid. Wind energy systems are gaining popularity for power generation as a form of “green technology” that does not consume fossil fuels, but instead converts wind-generated power for provision to electrical power distribution grids. DFIG-based converters adapt mechanical power generated by wind turbines to AC electric power in a form compatible with the grid, typically including a rotor driven by a turbine through a gearbox to supply power to a grid via stator connections. The DFIG rotor windings are connected to a back-to-back converter that includes a rotor side converter connected between the rotor windings and a DC circuit, along with a grid side converter connected between the DC circuit and the grid.
The DFIG system provides power from the DFIG stator windings to the grid with the DFIG rotor frequency often deviating from a nominal value corresponding to the grid frequency. DFIG converters essentially operate in one of two modes, depending on the rotating speed of the rotor. For rotor speeds below the nominal rotational speed, some of the stator power is fed to the rotor via the converters, with the grid side converter stage operating as a rectifier to supply power to the intermediate circuit and the rotor side converter inverting the DC power to power the rotor windings. When the rotor speed is above the nominal value, rotor currents are used to power the intermediate circuit, and the grid side converter operates as an inverter to supply power to the grid.
The DFIG converter can also control the rotor currents to adjust the active and reactive power fed to the grid from the stator independently of the rotor speed, and the DFIG generator is able to both import and export reactive power. This capability advantageously allows the DFIG system to support the grid during severe voltage disturbances (e.g., grid voltage sag conditions). The DFIG architecture also allows the DFIG to remain synchronized with the grid while the wind turbine speed changes, where variable speed wind turbines use the energy of the wind more efficiently than fixed speed turbines. The DFIG generator is typically constructed with significantly more rotor windings than stator windings such that the rotor currents are lower than the stator currents. Consequently, a relatively small back-to-back converter can be used, having components sized for operation within a certain rotor speed range.
However, transient DFIG rotor voltages upon grid faults are higher than the stator and grid voltages, and thus the rotor side converter and the intermediate DC circuit are particularly susceptible to voltage transients caused by grid disturbances such as grid voltage sag fault occurrences and clearance of these faults. Also, DFIG converters are typically subject to high current peaks upon the occurrence and clearance of grid faults, where current spikes on the rotor windings may exceed three times the rated value, depending on the leakage inductance and rotor resistance, and may trigger hardware over-current and/or overvoltage protection circuits to trip the converter. In particular, high current can flow through anti-parallel diodes in the rotor side converter even when the corresponding rotor side switching devices are off. If this happens, the DFIG system is not controlled during the critical time of grid faults, and thus the system cannot support the grid.
Previous attempts to address DFIG grid fault ride through issues included use of AC crowbar circuits on the rotor side to shunt current away from the rotor side converter during grid faults, but if the crowbar resistance is too low, current peaks on the rotor side may still be excessive. Moreover, if the crowbar resistance is too high, transient voltages can still charge the DC bus resulting in high charging current flowing through the anti-parallel (free-wheeling) diodes of the rotor side converter even if the corresponding converter switching devices are off. Other attempts involve insertion of damping resistance in series with DFIG stator windings, which can shorten the duration of the transient, but has little effect on reducing rotor side current peaks. Consequently, a need remains for improved DFIG converters and techniques to avoid or mitigate loss of DFIG system control for ride through of grid faults, particularly for low voltage ride through (LVRT).